readme: document cross-compiling for end users

Clarify the builtin cross-compiling support is meant for libweave
developers only.

BUG=b/26741388

Change-Id: I81c0d897d6b2f5d257ab9d50d2f96bf3d0a5448f
Reviewed-on: https://weave-review.googlesource.com/3070
Reviewed-by: Alex Vakulenko <avakulenko@google.com>

README.md

diff --git a/README.md b/README.md
index 428b0f5..c179e61 100644
--- a/README.md
+++ b/README.md
@@ -128,6 +128,29 @@
 
 ### Cross-compiling
 
+#### libweave users
+
+In order to cross-compile, all you need to configure is CC/CXX/AR.
+
+```
+make CC=your-cc CXX=your-cxx AR=your-ar
+```
+
+So if you have a toolchain in a path like `/opt/vendor/bin/arm-linux-gcc`, do:
+
+```
+make \
+  CC=/opt/vendor/bin/arm-linux-gcc \
+  CXX=/opt/vendor/bin/arm-linux-g++ \
+  AR=/opt/vendor/bin/arm-linux-ar
+```
+
+#### libweave developers
+
+*** note
+**Note:** This is only for developers who are hacking on libweave itself.
+***
+
 The build supports transparently downloading & using a few cross-compilers.
 Just add `cross-<arch>` to the command line in addition to the target you
 want to actually build.